The present invention relates to an image display device of the type which is mounted on portable equipment (for example, a mobile phone) or the like, and, more particularly, the invention relates to a technique which is effective at the time of automatically adjusting a common voltage applied to common electrodes in such an image display device.
A TFT (Thin Film Transistor) type liquid crystal display module having a miniaturized liquid crystal display panel, which is capable of producing a color display having a pixel arrangement of 100×150×3 pixels, is popularly used as a display part of portable equipment, such as a mobile phone.
FIG. 10 is a block diagram showing the circuit constitution of a conventional TFT type liquid crystal display module. As shown in the drawing, the conventional liquid crystal display module is constituted of a liquid crystal display panel 100, a display control device 110, a power source circuit 120, a drain driver 130 and a gate driver 140.
FIG. 11 is an equivalent circuit diagram of one example of the liquid crystal display panel 100 shown in FIG. 10. As shown in FIG. 11, the liquid crystal display panel 100 includes a plurality of pixels arranged in a matrix array. Each pixel is arranged a region bounded by two neighboring signal lines (drain signal lines D or gate signal lines G) and two neighboring signal lines (gate signal lines G or drain signal lines D). Each pixel includes a thin film transistor (TFT), and a source electrode of the thin film transistor (TFT) of each pixel is connected to a pixel electrode (ITO1).
Further, since a liquid crystal layer is provided between the pixel electrode (ITO1) and a common electrode (also referred to as a counter electrode) (ITO2), a liquid crystal capacitance (CLC) is equivalently connected between the pixel electrode (ITO1) and the common electrode (ITO2). Still further, between the source electrode of the thin film transistor (TFT) and the common electrode (ITO2), a storage capacitance (CS) is connected.
In the liquid crystal display panel 100 shown in FIG. 10, the drain electrodes of the thin film transistors (TFT) of respective pixels, which are arranged in the column direction, are respectively connected to the drain signal lines (also referred to as video signal lines) D, and the respective drain signal lines D are connected to the drain driver 130, which applies gray scale voltages to the liquid crystal of respective pixels in the column direction.
Further, the gate electrodes of the thin film transistors (TFT) of respective pixels, which are arranged in the row direction, are respectively connected to the gate signal lines (also referred to as scanning signal lines) G, and the respective gate signal lines G are connected to the gate driver 140, which applies a scanning driving voltage (a positive bias voltage or a negative bias voltage) to the gate electrodes of the thin film transistors (TFT) of respective pixels in the row direction for one horizontal scanning period.
The display control device 110 controls and drives the drain driver 130 and the gate driver 140 in response to respective display control signals, including clock signals, display timing signals, horizontal synchronizing signals and vertical synchronizing signals, and display data (R, G, B) which are transmitted from the outside.
The power source circuit 120 supplies a gray scale reference voltage to the drain driver 130 and, at the same time, supplies a scanning driving voltage to the gate driver 140 and, further, supplies a common voltage to the common electrodes (ITO2). Further, the power source circuit 120 supplies a power source voltage for the drain driver 130 and the gate driver 140 to the drain driver 130 and the gate driver 140.
The gate driver 140 sequentially supplies a scanning signal voltage, which turns on the thin film transistor (TFT), to the gate signal lines G one after another for every one horizontal scanning period, thus turning on the thin film transistors (TFT).
Further, the drain driver 130 supplies a video signal voltage to the drain signal lines D and applies the video signal voltage to the pixel electrodes (ITO1) through the thin film transistors (TFT) which are turned on, writes the video signal voltage into the respective pixels, and charges a given voltage to the liquid crystal capacitances (CLC) between the pixel electrodes (ITO1) and the common electrodes (ITO2).
The orientation directions of liquid crystal molecules of respective pixels are changed based on the charged voltage so as to display an image. In accordance with the above-mentioned operations, an image is displayed on the liquid crystal display panel 100.
Here, when a DC voltage is applied to the liquid crystal, the lifetime of the liquid crystal becomes short. To prevent such a phenomenon, in the liquid crystal display module, the voltage applied to the liquid crystal layer is alternated every fixed period. That is, the voltage applied to the pixel electrodes is changed to the positive voltage side (hereinafter referred to as a gray scale voltage of positive polarity) and the negative voltage side (hereinafter referred to as a gray scale voltage of negative polarity) with respect to the voltage applied to the common electrodes, which are used as the reference for every fixed period.
In the above-mentioned constitution, it is ideal that the voltage applied to the liquid crystal at the time of writing is held until the next writing takes place. However, in an actual operation, as indicated by the dotted line in FIG. 11, there exists a floating capacitance (CGS) between the source and gate of the thin film transistor (TFT); and, hence, after the thin film transistor (TFT) is turned off, the voltage of the pixel electrodes is changed due to the floating capacitance (CGS). A voltage change quantity ΔV attributed to the floating capacitance (CGS) is expressed by a following formula (1).ΔV=CGS/(CLC+CGS)×ΔVG  (1)Here, ΔVG indicates the difference between the gate voltage when the thin film transistor (TFT) is in an ON state and the gate voltage when the thin film transistor (TFT) is in an OFF state.
In this manner, the voltage (that is, the voltage of the pixel electrodes (ITO1) which is actually held in the liquid crystal) is changed from the liquid crystal applied voltage which is applied to the drain signal lines (D) by ΔV.
Here, although the voltage of the pixel electrodes (ITO1) is also changed due to the influence of other floating capacitances, an explanation is made with respect to only the floating capacitance CGS between the gate and the source of the thin film transistor (TFT), since it exerts the largest influence on the voltage of the pixel electrodes (ITO1).
Further, although the voltage (Vcom) which is applied to the common electrodes (ITO2) is originally to be set to a center value of the liquid crystal applied voltage, since the voltage of the pixel electrode (ITO1) is changed in response to the liquid crystal applied voltage by ΔV, the potential difference between the voltage of the pixel electrode (ITO1) at the time of positive polarity and the voltage (Vcom) of the common electrodes and the potential difference between the voltage of the pixel electrode (ITO1) at the time of negative polarity and the voltage (Vcom) of the common electrodes differ from each other; and, hence, an asymmetrical voltage is applied to the liquid crystal with respect to the voltage (Vcom) of the common voltage (ITO2) between the case of positive polarity and the case of negative polarity.
When such an asymmetrical voltage is applied to the liquid crystal, flickers are generated on the screen. For example, in producing a display using a signal source having the vertical synchronizing signal of 60 Hz, when a voltage of the same polarity is applied to all neighboring pixels and the polarity of the voltage is inverted for every one screen, the polarity of the voltage is changed at a cycle of 30 Hz. That is, the asymmetrical voltage is held in the liquid crystal at a cycle of 30 Hz and the brightness is changed by an amount corresponding to the voltage difference; and, this change of brightness is observed as flickers.
Accordingly, it is necessary to adjust the voltage (Vcom) applied to the common electrodes (ITO2) in response to the above-mentioned voltage change quantity ΔV. However, the required adjustment quantity differs delicately for respective products (LCD), and, hence, it is necessary to perform a specified adjustment for respective liquid crystal panels.
In general, as methods for adjusting the voltage (Vcom) applied to the common electrodes (ITO2), there have been known a method in which an operator manually performs the adjustment by confirming an actual state of flickers on a liquid crystal panel and a method which automatically performs the adjustment.
In the manual adjusting method, the voltage (Vcom) applied to the common electrodes (ITO2) is generally adjusted by changing the resistance value of a variable resistance. In this case, a method which facilitates the adjusting method is described in Japanese Unexamined Patent Publication Hei8(1996)-63128 (patent literature 1).
Further, with respect to the automatic adjusting method, Japanese Unexamined Patent Publication Hei10(1998)-246879 (patent literature 2) and Japanese Unexamined Patent Publication Hei8(1996)-286169 (patent literature 3) describe a method in which dummy pixels are provided, a specific gray scale voltage is applied to the dummy pixels, light emitted from the dummy pixels is converted into a voltage by light receiving elements, and a voltage (Vcom) applied to common electrodes (ITO2) is adjusted based on the voltage.